The US Air Force is preparing for the imminent landing of its second secret spaceplane, which is expected sometime at the beginning of June.

The reusable X-37B, an unmanned shuttle with an unknown mission, will probably come down at Vandenberg Air Force Base in California, although Edwards Air Force Base in the same state is a backup landing site.

The Space Wing Public Affairs staff at Vandenberg said that the exact landing date and time would "depend on technical and weather considerations", but it was expected "during the early- to mid-June timeframe".

"The men and women of Team Vandenberg are ready to execute safe landing operations anytime and at a moment's notice," Colonel Nina Armagno, the 30th Space Wing commander said in a canned statement.

The X-37B is a mysterious, black-funded robot shuttle that no-one knows the purpose of. The Air Force does its best to convince folks it's just a reusable craft for testing space tech, but its clandestine nature has led to all sorts of speculation.

The Iranians call the ship a "secret space warplane", while amateur space trackers have said that the craft is clearly spying on China's space laboratory Tiangong-1. ®